An intersected station is one at which the coordinates were derived by intersection, no observations were ever taken at the station. Intersection is a process where rays are observed to the station from many outside points and the coordinates derived from the resulting intersection of those rays
The main distinguishing factor is that intersected stations generally cannot be occupied by a theodolite.
However there is the noticeable exception of St Pauls Cathedral. This was originally intersected during the Principal triangulation and later (during the Principal) a scaffold was erected and the Ramsden 36" theodolite taken to the top and observations taken. .
Examples of Intersected marks include Centres, church spires, weather vanes, pinnacles and chimneys. They were often used in conjunction with a nearby mark that could be 'occupied', for example a bolt on the church roof. The station name is only classified as Intersected in the OSGB36 list if it does not include any occupiable marks. If there is a mix of occupiable marks (i.e rivet or bolt) and non-occupiable (centre, church spire, vane, flagstaff, etc) then it is not an Intersected station.
The following table is extracted from . 'count1' counts records where the station name contains "I" (denoting that it comprises only 'Intersected' marks). 'count2' counts records where the the station comprises a mix of Intersected and Occupiable marks.
The inclusion of a block as an intersected station may seem surprising. The single instance is Lochboisdale Pier for which the OSGB36 record states "Centre of conc block on pier". This suggests that it was not an OS block with a metal mark but a pre-existing non-OS concrete block with a mark type of 'Centre'.
- Email from Ian Wilson to the Trigonomy mailing list, 4 July 2012